SPRINGS 



141 



HELICAL SPRINGS OF GIVEN LOAD RATIO AND LENGTH RATIO 



Graphical computation charts, developed by Frederick Franz, for springs for 

 specified maximum load and length and specified minimum load and length based on 



G = 11,500,000 S = 50,000 



Step 1. To determine spring index. 



Compression Springs. — Divide specified initial load Pmm. on spring by maximum 

 load Pmax., when compressed, to obtain load ratio. Similarly, calculate length ratio 

 of compressed length L„,in. to initial length Lma^.- The intersection of the vertical 

 line representing load ratio and the horizontal line representing length ratio gives 

 value of D/d, the ratio of outside diameter of coil to the diameter of the wire. 



Tension Springs. — Divide initial tension on spring by final tension, to obtain 

 load ratio. Divide initial length of spring by maximum length, to obtain length ratio. 

 The intersection of the vertical line representing load ratio -with the horizontal line 

 representing length ratio gives D/d, the ratio of outside diameter of coil to the diameter 

 of the wire. 



0,8 



0,7 



0.6 



05 



0.4- 



0.3 



0,2 



0.1 

 



0.1 Q2 0,3 0,4 0,5 0.6 0.7 Q8 09 1,0 



Ql 0,2 03 0,4 0.5 06 07 Q8 0.9 1,0 



Length of a compression spring must be calculated as the length of active spring, 

 i.e., total length less length taken by inactive coils, namely, \}i dead turns at each end. 

 For compression springs, a minimum clearance of O.ld (wire diameter) must be allowed. 

 For example, if the space available for a compression spring is 6 in. and a rough esti- 

 mate indicates a wire diameter of Vi in., and Wi dead turns on each end, the maxi- 

 mum length of spring will be 6 in. less the length equivalent of 2K turns which is Y?, in. 

 Thus, Lmax. would be 5^8 in. If the minimum to which this spring is to be compressed 

 is to be 5 in., the minimum active length will hkewise be 5 in. less ?8 in., or 4^8 in. 

 approximately. 



Step 2. To determine maximum safe load, wire diameter, and mean coil diam- 

 eter for hehcal round wire tension or compression springs; based on 50,000 lb. per 

 sq. in. allowable stress. 



